Where to put the stack boxes is a question we debated for quite a while before
we made our decision for K1IR. The trade-off is primarily cable loss and cost
[coax and control] versus convenience and maintainability. There are really
three or four locations to consider:
1. Each switchbox at the midpoint on the tower between the antennas in the
stack.
- Coax: Minimum toal coax needed due to shortest length of phasing lines
- Control cable: Longest total run
- Convenience: Tower must be climbed to make any change or perform any
test or repair
- Maintainability: Box fully exposed to weather. Slightly lower tower
windload due to less cables running up the full length of the tower.
2. Switchboxes at the base of the tower - mounted to tower legs.
- Coax: More coax needed due to increased length of phasing lines
- Control cable: Somewhat shorter total run
- Convenience: Outdoor access, and tower need not be climbed to make any
change or perform any test or repair
- Maintainability: Boxes fully exposed to weather
3. Switchboxes at the base of the tower - in a weatherproof enclosure.
- Coax: Same as option 2
- Control cable: Same as option 2
- Convenience: Outdoor access, and tower need not be climbed to make any
change or perform any test or repair
- Maintainability: Boxes not exposed to weather
4. Switchboxes for multiple stacks on multiple towers combined in a
ground-mounted, weatherproof enclosure at a central point in the antenna field.
- Coax: More coax than options 2 and 3 [longer phasing lines]
- Control cable: Similar to options 2 and 3
- Convenience: Outdoor access, and tower need not be climbed to make any
change or perform any test or repair. All control cables and main feeds can be
run together to the enclosure.
- Maintainability: Boxes not exposed to weather
5. Switchboxes for multiple stacks on multiple towers located in the shack.
- Coax: Much more coax than options 2, 3 and 4 [very long phasing lines]
- Control cable: Almost none
- Convenience: Easy, indoor access inside the shack
- Maintainability: Boxes not exposed to weather
So, how do you decide which way to go? The primary factor to consider is the
amount of coax required. You need to have a budget for total coax loss and
cost. When all antennas in the stack are operating, you are supplying power to
all the coax in the system. So, just calculate the total coax loss at the
frequency of interest in all of the runs and you will know how much power you
are losing in the system. Big difference between 40m and 10m. Consider the cost
impact versus the performance improvement you would gain with lower loss coax.
In my case, I mounted the 20/15/10m stack switches to the legs at the base of
the tower. I used RG-213 for all coax runs. The convenience and performance has
been excellent, but weather wear has been significant. If I were to do it again
[and I just might], I would mount a large equipment box on a panel near the
base of the tower and protect the switch boxes from the weather. This box would
also house lightning protection and other useful stuff.
Jim Idelson K1IR
email k1ir at designet.com
web http://www.k1ir.com
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